This invention relates to an apparatus for automatically cleaning the electrodes of a pH meter so as to effectively remove scale adhering to the surface of the electrodes of the pH meter.
Purification section for removing impurities in a leaching solution supplied from a leaching section of electritic zinc plant comprises a first, a second and a third stage. Zinc dust is used for removing the impurities such as copper, nickel, cadmium etc. A reagent such as .alpha.-nitroso-.beta.-naphthol (the so-called N-N reagent) is commonly used for the removal of cobalt contained in the leaching solution. It is a matter of great importance to properly control the pH value of this solution. It has been common practice to use a test paper for the purpose of this pH control. This manner of pH control has however been defective in that the pH value cannot be accurately measured and discontinuous manual measurement results in an unsatisfactory pH control. Continuous measurement of the pH value by a process pH meter has been proposed in an effort to obviate the defect resulting from the manual measurement of the pH value by the test paper. This proposal has however been also defective in that scale adheres to the glass surface of the glass electrodes of the pH meter in several hours after the commencement of measurement thereby providing a hindrance to the desired accurate and reliable measurement of the pH value.
In order to carry out the desired continuous measurement with such a pH meter, the scale must be effectively removed without adversely affecting the function of the electrodes of the pH meter. However, the scale adhering to the surface of the glass electrodes is in the form of a gummy matter formed by polymerization of .alpha.-nitroso-.beta.-naphthol and is generally quite adhesive to such an extent that it can be hardly removed once adhered to the surface of the glass electrodes. While brushing has been found most suitable for the effective removal of such adhesive scale, automatic brushing means suitable for use for the electrodes of the kind above described have not been developed up to the present. This is principally due to the difficulty of assembly of automatic brushing apparatus in the pH meter for the following reasons among others: Firstly, the electrodes of the pH meter are small in size and covered with a thin glass film. Therefore, the electrodes have not a sufficient mechanical strength to withstand the cleaning force. Secondly, careful handling is required since the pH meter is a highly precise and sensitive measuring instrument. Thirdly, the electrodes must be periodically taken out of the pH meter for maintenance.